This invention relates to an improvement in speedometer drives and, in particular, to an apparatus forming an element of a speedometer-odometer assembly which transfers and translates the rotation of the speedometer drive cable to the remaining elements of the speedometer-odometer assembly.
Conventional speedometer-odometer assemblies for recording the speed and distance traveled by a moving vehicle are fairly simple mechanical devices including a threaded or worm drive shaft mounted on a frame and connected for rotation at one end with a speedometer drive cable being rotated by the revolving wheels of the vehicle, and having disposed at its opposite end a magnet which rotates with it. When the shaft is rotated by the speedometer drive cable, the threaded portion of the shaft engages the teeth of a crossover gear running substantially perpendicular to the shaft and imparts a rotation to this crossover gear. At the opposite end of the crossover gear are disposed circumferential threads, which engage and rotate, in a similar manner, the teeth at one end of an odometer worm drive gear. This odometer worm drive gear has threads at its opposite end engaging teeth disposed about the circumference of one of a series of numeraled odometer drums which register the revolutions of the wheels of the vehicle as distance for the benefit of the driver of the vehicle.
The magnet located at the forward or driving end of the rotating threaded shaft creates, upon rotation, a magnetic flux which causes a spring-biased needle registering the speed of the vehicle to be held in a certain position with respect to a reference point, depending upon the speed of rotation of the speedometer cable and connected shaft.
The use of a series of gears in the odometer assembly permits a significant reduction from the speed of rotation of the speedometer drive cable to the speed of rotation of the odometer numeral drum. The greater this gear reduction, of course, the lesser the chances of mechanical malfunction of the assembly and the greater the accuracy of the readings.
U.S. Pat. No. 3,739,653 improved on the above-described speedometer-odometer drive assemblies and, in particular, in the worm drive or threaded shaft element responsible for transferring the rotation of the speedometer drive cable to the odometer gear train and for translating the rotation of this cable to the speedometer needle to register speed. This improvement eliminated the necessity of machining this particular worm drive element and, therefore, reduced the time and expense necessary to produce this element and the overall speedometer-odometer assembly without affecting the quality and operation of the finished product. This worm drive apparatus accomplished the same function as more expensive machined or cylindrically-turned worm drive elements by an annular plate means or disc disposed on a drive shaft connected to the speedometer drive cable and rotatable therewith. This disc is split or cut from its periphery toward its center at at least one point about its circumference and the facing edges of the cut are one or both upset or separated with respect to the radial plane of the surface of the disc. The outside edge or periphery of the disc engages the teeth of a conventional odometer drive worm element of a speedometer-odometer assembly in such a manner that when the speedometer cable imparts a rotation to the drive shaft and the disc, the edge of the disc at the point of upset engages successive teeth on the odometer worm drive, thereby causing it to rotate in the same manner as in a conventional drive train.
In addition, the annular plate means, which substitutes for the threaded portion or worm gear of the prior apparatus, may also be shaped in its surface, without machining, to receive the magnetic drive means necessary to produce the magnetic field driving the spring-biased speedometer needle. This is done by cutting or forming two flanges from the surface of the disc and bending these flanges perpendicular to the surface on opposite diameters so that a magnet may be placed between them, while providing a definite space between the inside surface of each flange and the end of the magnet. In this position, each flange allows the flux cup disposed over the magnet to pass between it and the end of the magnet as the flanges rotate with the disc about the circumference of the flux cup. Thus, the disc not only drives the odometer, but also serves to drive the speedometer.
While these above-described improvements aided in reducing costs associated in the gear train for transferring the rotational movement of the speedometer cable to the speedometer-odometer assembly, it has been found that the odometer worm drive of the prior art must necessarily be loosely engaged with the annular plate in order to effect the free rotation of the worm drive and the odometer. This loose engagement can lead to increased wear in the odometer drive train, more specifically, in the gear teeth. A jar or vibration encountered by the assembly can cause the worm gear to rotate to a position slightly out of engagement with the annular plate. The continued rotation of the annular plate forces the worm gear into correct alignment, but at the cost of worn teeth, wear and an annoying "clicking" sound, both problems caused by the annular plate making a sudden clash with the worm gear. Of course, the worn teeth worsen the loose gear engagement, causing even more noise.
The odometers of the prior art also have no commercially acceptable manner to prevent the backward-driven rotation of the odometer. In fact, the act of proceeding in reverse causes the odometer drive and the odometer display to also proceed in reverse, thereby causing error in the odometer reading as to the total amount of distance traveled.